Asked whether he would be acting prime minister when Malcolm Turnbull heads to the Pacific Islands Forum in Samoa at the end of the week, Mr Joyce said: "That's how the game works. The PM always has that right."

Labor argues there's no way the Nationals leader should be acting prime minister while his eligibility is in doubt.

The Greens also want Mr Joyce to stand down, tying the issue to the proposed Adani mine in Queensland.

"A minister who's under a legal question mark should not be making decisions to give a billion dollars of taxpayers' money to companies under a legal question to light a fuse on a climate time bomb," MP Adam Bandt said.

But independent senator Nick Xenophon, who faces his own referral to the High Court, queried whether Mr Joyce was doing anything illegal.

"Until the High Court determines otherwise, he can still keep doing his job," he told reporters.

Former prime minister Tony Abbott came to the defence of Mr Joyce, insisting his case was the same as Labor leader Bill Shorten's, whose father was also born overseas.